An electric lamp having a lead wire construction of improved stiffness sufficient to eliminate the need for additional support of the incandescent filament is disclosed in U.S. Pat. No. 4,131,819, also assigned to the present assignee. In said issued patent, the inlead wires are said to exhibit a stiffness value within the range of approximately 300-500 in order to eliminate using tie wires in the lamp without sacrificing the further need of shock resistance. The improved lamp construction illustrated therein includes a specific mount construction for hermetic sealing of the inleads to the outer glass envelope of the lamp in the form of an outer hollow glass tube having flare portion for sealing to the glass envelope which further includes an inner exhaust tube also generally of glass material. The illustrated lamp embodiment still further includes suspension of the resistive incandescent filament by the inleads alone in a transverse direction with respect to the longitudinal direction of said wires and which is customarily termed a CC6 mount orientation of said filament. A different filament orientation is also known although not specifically illustrated in said patent wherein the longitudinal direction of said filament is aligned in the same direction as the longitudinal direction of the inlead wires and with said arrangement being termed a CC8 mount construction.
Use of the commercially available dispersion-strengthened copper alloy inlead wire occasions embrittlement of a tungsten lamp filament when said inleads are nickel-plated in the customary manner. Specifically, this serious problem causing premature lamp failure occurs when nickel migrates to the tungsten filament generally from a location near the point of interconnection between the supporting inleads and the filament. While this cause of filament brittleness has been encountered previously in incandescent lamps utilizing other nickel-plated copper alloys, the lamp failures experienced when dispersion-strengthened copper inleads are nickel-plated to provide the sole structural support for the lamp filament are considerably more frequent. It has thereby not been possible to realize the full benefits in lamp construction from a substitution of dispersion-strengthened copper alloys as the inlead material since the customary nickel-plating of this material results in unreliable lamp performance.